Three-way switches control an electrical load from two locations. Depending on the location of the light (whether it falls between the switches or after both), the wiring sequence will differ.
To learn how to wire a three-way switch when both switches come after the fixture, and sort out other finish-wiring head-scratchers, visit the JLC Field Guide.
For more about the changes to the 2014 National Electric Code requiring an available neutral, see “Neutral Necessity: Wiring Three-Way Switches.”